Although casein glues and animal glues have been used in the past to glue wood, the most commonly used glues today for wood assembly applications are the so-called white glues and/or specialty formulated carpenter glues which are based on polyvinyl acetate homopolymers or polyvinyl acetate copolymer emulsions which may or may not contain cross-linking functionality. Some manufacturers use the white glues in edge gluing and in finger-joint bonding. Still others use these adhesives for laminating materials, such as veneer or plastics to the core of flush doors, wood or particle board.
White glues or carpenter glues stand up best in supported joints, such as dowel joints, dado joints, or mortise and tenon joints. They are not usually used in butt joints, especially if the joint must bear weight. This is because polyvinyl acetate homopolymer or copolymer is thermoplastic when not cross-linked completely, and if the bond in a butt assembly should deform because of cold flow, the assembly would slide apart.
Wood glues, including the white glues, are judged by their ability to bind to wood fiber. Ideally, the wood fiber tears before the glue bond fails. A popular on-line test for a wood glue requires that the glue bind sufficiently in 1 to 3 minutes and that there be 60% to 80% wood fiber tear when the bond is broken.
Early white glues or carpenter glues contained from 95 to 98 percent polyvinyl acetate emulsion, with the remainder made up of a plasticizer. They were sold for general-purpose household use as well as for wood bonding. However, today's commercial wood glues typically contain a polyvinyl acetate homopolymer and/or polyvinyl acetate copolymer, a polyvinyl alcohol solution, a defoamer, a plasticizer, and a preservative. They may also contain cross-linking capability, solvents, fillers or other ingredients known to those practiced in the art. These adhesives may be supplied as one or multiple component systems which may be blended prior to application in various ratios depending on specific end-use performance and "pot life" requirements.
A cross-linking catalyst for wood glue which has been used in the past was aluminum chloride (AlCl.sub.3) Unfortunately for many wood assembly applications, glues containing aluminum chloride as the catalyst did not pass the "on-line" test for wood fiber tear. In addition, the resulting bond was not as heat stable or water resistant as desired. Various other cross-linking agents have been employed since but with only limited success.
Many glues commercially available for wood gluing applications work well in the laboratory, but they do not work well at the ambient temperatures encountered in the wood working factories, which can range from as low as 35.degree. F. to as high as 120.degree. F. In addition some of the wood glues do not possess the pot life desired which ranges from 18-24 hours.
It would be advantageous to have an adhesive that would bond wood faster and perform well at ambient temperatures encountered in a wood working factory. It also would be advantageous to have an adhesive which provides a more water and heat resistant bond than present wood glues.